Not Applicable
Not Applicable
1. Field of Invention
This invention relates, in general, to semiconductor photodiodes, and more specifically to a semiconductor photodiode having one electrical contact that extends from a p-n junction at the surface of the photodiode to the back of the photodiode substrate and a second electrical contact located of the back of the photodiode substrate.
2. Description of Prior Art
A photodiode detector is broadly defined as a device that responds to incident electromagnetic radiation by converting the radiation into electrical energy, thereby enabling measurement of the intensity of the incident radiation. Usually, a photodiode structure requires a small wire to be bonded to the electrical contact on the top surface of the photodiode. This wire must extend above the photodiode surface and only touch the surface at the electrical contact point. These wires (normally referred to as wire bonds) are very delicate, prone to damage and limit the proximity that a photodiode can be to a surface. In some applications, such as x-ray, extreme ultraviolet and deep ultraviolet (XUV radiation) steppers, it is desirable to place the detector within a few micrometers to a given surface for dosimetry, imaging, position sensing and alignment applications. Wire bonds on the front surface of the photodiode, which typically are 1 millimeter high, will not allow this.
Photodiodes used as solar cells have been manufactured with electrical contacts on the back surface opposing the front active area (back contact devices) (citation #1). According to Smith et. al. (citation #2), there are three design categories of back contact cells, the Interdigitated Back-Contact (IBC) solar cell, the Emitter Wrap-Through (EWT) solar cell, and the contact wrap-through (CWT) solar cell.
Both the IBC solar cell and the CWT solar cell are not suitable designs for making a photodiode for XUV radiation detection. The IBC solar cell does not have an electrical contact that extends to the front surface of the diode which is needed for XUV photon detection. It is essential for XUV photodiodes to have the p-n junction formed near the surface as XUV radiation is absorbed within a fraction of a micron from the surface. The CWT solar cells have a metal grid on the cell surface which reduces the sensitivity of the cell. For many applications (mainly radiometric), it is desirable to have an XUV photodiode with very high sensitivity without any radiation absorbing structure on the active surface. Although the CWT solar cells have holes for front to back contact, the holes create an additional problem in realizing the XUV photodiodes as explained in the next paragraph. The buried contact EWT cell was one of the earliest versions developed. It uses laser drilled holes from front to back of the cell as well as laser cut channels for contact plating. The problem with this method is that the holes in the wafer will make it almost impossible to pattern wafers using photolithography techniques needed for the above mentioned applications. Several patterning steps are required to realize an XUV photodiode after formation of the front to back contact.
Fabrication of the back contact vertical junction solar cells have been described in U.S. Pat. No. 5,067,985 (citation # P1). Special (110) oriented silicon wafers were chosen and chemical etching was used to form a network of narrow channels into which impurities were diffused. Then a conducting material was deposited into the channels to form back contacts. (110) oriented silicon is useful only when long narrow channels are needed as in the solar cell structures. Small channel areas like those required in dense photodiode arrays can not be realized by wet chemical etching techniques.
We have made back contact photodiodes for x-ray steppers several years ago in which the holes in the wafer were formed by wet chemical etching through which the front contact was brought to the back of the photodiode. In this method, an area of about 0.75 mmxc3x970.75 mm is needed to achieve the desired results. Hence, the photodiodes on the front of the wafer can not be placed closer than about a millimeter to each other, an undesirable feature when a photodiode dense array is desired for imaging purposes.
An obvious approach to avoid the use of wire bonds would be to mount the front surface of the diode onto a ceramic substrate and illuminate the diode from the back (citation #3). High XUV response of back illuminated devices dictates that the devices need to be thin so that the photogenerated carriers can diffuse to the front region. However, radiation hardness as that required by XUV photodiodes, has so far, not been demonstrated by the thin devices (citation #4).
A brute force approach used in some silicon controlled rectifier (thyristor) manufacturing processes, is to carry out the dopant diffusion in desired areas from the front and back of the wafer simultaneously. When the front and back diffusions meet, they form a conductive channel extending from the front of the device to the back of the device. However, temperatures in excess of 1200xc2x0 C. and diffusion time in excess of a week are required to achieve the desired results. Such a high temperature and prolonged process time is known to mechanically deform the silicon wafers. If the wafers are deformed, it will be impossible to perform the submicron geometry patterning needed for the above XUV stepper applications.
Accordingly, it is an object of the present invention to provide a radiometric quality semiconductor photodiode with shallow p-n junction having one electrical contact through a small channel extending from the p-n junction to the back of the photodiode substrate and a second contact formed on the back of the semiconductor substrate. The electrical contact channel requires a cross sectional area of only 0.125 mmxc3x970.125 mm with the present technology and can be made smaller as the semiconductor etching technology develops further.
Another object of the present invention is to provide a method of manufacturing a radiometric quality semiconductor photodiode with shallow p-n junction having one electrical contact through a small channel extending from the p-n junction to the back of the photodiode substrate and a second contact formed on the back of the semiconductor substrate. In the method proposed, the electrical contact channel will require a cross sectional area of only 0.125 mmxc3x970.125 mm.
These and other objects and advantages are provided by a radiometric quality semiconductor photodiode with one electrical contact extending from a p-n junction at the photodiode surface to the back of the photodiode substrate and a second contact formed on the back of the semiconductor substrate. With the present technology, the electrical contact channel will require a cross sectional area of only 0.125 mmxc3x970.125 mm.
This photodiode with back contacts will eliminate the need for delicate wire bonds which are prone to damage during handling and will also allow the photodiode to be placed extremely close to a surface. Because of the small contact channel area of 0.125 mmxc3x970.125 mm, dense photodiode arrays can be manufactured.
A novel approach to accomplish this will require the formation of a channel by dry etching that will extend from back of the photodiode substrate to close to the p-n junction at the front of the photodiode. Impurities will then be diffused radially outward from the channel to provide an electrical pathway from the p-n junction to the back of the substrate. Dry etching will provide precise control of channel depth close to the photodiode front surface which would not be achievable with laser drilling as used in the prior art. This will provide a means for manufacturing a radiometric quality photodiode suitable for the previously mentioned applications.
According to one embodiment, the present invention comprises a radiometric quality semiconductor photodiode with shallow p-n junction having one electrical contact through a small channel extending from a p-n junction at the photodiode surface to the back of the photodiode substrate and a second contact formed on the back of the semiconductor substrate.
In another embodiment, the present invention comprises a method of making a radiometric quality semiconductor photodiode with shallow p-n junction having one electrical contact with a small channel extending from a p-n junction at the photodiode surface to the back of the photodiode substrate and a second contact formed on the back of the semiconductor substrate. The method comprises fabrication of a radiometric quality semiconductor photodiode having one electrical contact extending from a shallow p-n junction to the back of the photodiode substrate and with a second contact formed on the back of the semiconductor substrate by using techniques of photolithography and dry etching presently used in the semiconductor industry.